Fail passive vs fail operational on 737NG

[N1 Limit]

Hi there

Is there anyone to help me really understand the above mentionned type of approaches and How they're really conducted.Thanks in advance

[Advs]

Fail Operational. 3 autopilots are used on a fully auto landing. If one fails then two still allow the landing to be continued. Hence when three work it's fail operational.
If one has already failed the system becomes fail passive meaning another failure would result in the auto land no longer allowed. As only one auto pilot remains.

That's from ATPL systems. Don't quote me but it's my best idea of it.

[N1 Limit]

Then i assume on the 737 NG there's no fail passive as there are Only 2 autopilots.

[Denti]

ATPL knowledge is pretty old and inaccurate. Even the A320 has only two autopilots and is fail operational. The 737NG with a fail operational autoflight system (which is different from the fail passive one) has only two autopilots as well.

But what exactly do you want to know?

[ImbracableCrunk]

Other than the rollout, the two are conducted the same way, you just have different indications in the cockpit. On the NG Fail Passive, you get Single Channel when the LOC caps and then it turns to CMD below 1500'. On the Fail Op, it goes from Single Channel to LAND3 in a normal situation.

There's more to it, but basically punch approach, punch the 2nd A/P.

[OutofSync]

On B737NG aircraft fitted with fail operational systems electrically operated rudder actuators, which receive inputs from their respective FCC, provide an alignment and rollout capability not present in fail passive aircraft. Rudder pedal movement and nose-wheel steering are provided by feedback through the rudder flight control system. Additional inertial information for the fail operational capability is provided by the Integrated Standby Flight Display (ISFD).

The ISFD provides the redundancy that a 3rd Autopilot would normally provide. The approach system failure matrix for fail operational equipped aircraft (contained in appropriate AFM) is slightly different than that for fail passive aircraft. One example is that if an engine fails on approach the dual channel approach and landing may be continued in a fail operational aircraft.

The fail operational system allows operation in CAT IIIB conditions if the operator and crew are approved. Whereas the fail passive system is restricted to CAT IIIA operations.

[N1 Limit]

Thanks a bunch guys for all your inputs,it's highly appreciated

[Denti]

@OoS, our documentation only tells us tha the ISFD is a third attitude source, but not a monitoring unit. Do you have any documentation about additional stuff? A third autopilot is usually used for an automatic two out of three solution, but that is quite a bit more than just attitude information.

Anyway, the nice thing is that you do not need a take off alternate anymore as the fail operative NGs can do single engine CAT III approaches, and of course the max crosswind component is increased to 25kts for normal operation. The automatic rollout works like a charm and the fail operational autoland does a pretty good job as well, especially with a bit of wind.

[bedix84]

Fail-passive Automatic Landing System
An automatic landing system is fail-passive if, in the event of a failure, there is no significant out-of-trim condition or deviation of flight path or attitude - but the landing is not completed automatically.
NOTE: For a fail-passive automatic landing system the pilot assumes control of the aircraft after a failure.
The following are typical arrangements:
(1) A monitored automatic pilot in which automatic monitors will provide the necessary failure detection and protection. (2) Two automatic pilots with automatic comparison to provide the necessary failure detection and protection.
Fail-operational Automatic Landing System.
An automatic landing system is fail-operational if, in the event of a failure, the approach, flare and landing can be completed by the remaining part of the automatic system.

NOTE: In the event of a failure, the automatic landing system will operate as a fail-passive system.
The following are typical arrangements:
(1) Two monitored automatic pilots, one remaining operative after a failure. (2) Three automatic pilots, two remaining operative (to permit comparison and provide necessary failure detection and protection) after a failure.
Fail-operational Hybrid Landing System
A system which consists of a primary fail-passive automatic landing system and a secondary independent guidance system enabling the pilot to complete a landing manually after failure of the primary system.

[iaveight]

There is a fail passive on the B737 - but it depends on the malfunction

A flashing A/P light below 800 feet indicates the auto pilot has disengaged , a steady A/P light indicates the stab is out of trim

A flashing A/T light means the auto throttle has disengaged

They both flash red in this instance , flashing amber could mean the test function is being held

But this is fail passive - no sig deviation of aircraft flight path or attitude

[cap.miko]

@OoS, our documentation only tells us tha the ISFD is a third attitude source, but not a monitoring unit. Do you have any documentation about additional stuff? A third autopilot is usually used for an automatic two out of three solution, but that is quite a bit more than just attitude information.

Anyway, the nice thing is that you do not need a take off alternate anymore as the fail operative NGs can do single engine CAT III approaches, and of course the max crosswind component is increased to 25kts for normal operation. The automatic rollout works like a charm and the fail operational autoland does a pretty good job as well, especially with a bit of wind.

Chk FCTM 5.31